1. Field of the Invention
The present invention relates to an optical recording apparatus such as a laser beam printer for performing optical recording by scanning and modulating laser beams.
2. Background Art
As examples in the related art, there are Japanese Patent Laid-Open No. 101112/1992, Japanese Patent Laid-Open No. 15625/1996 and Japanese Patent Laid-Open No. 109458/1997.
Japanese Patent Laid-Open No. 101112/1992 has disclosed a multibeam scanning optical system including: a multibeam light source constituted by a plurality of semiconductor lasers arranged in a row in a subsidiary scanning direction; a rotary polygon mirror for deflecting multibeam light from the light source and scanning the multibeam light on a photosensitive drum; and an adjusting member constituted by two lens groups, namely a front lens group and a rear lens group, and provided in an optical path between the multibeam light source and the rotary polygon mirror; wherein the distance between the front lens group and the rear lens group is relatively changed to change the focal lengths of the two lens groups to thereby change image-forming magnification and adjust a light beam interval in the subsidiary scanning direction on the photo sensitive drum.
Japanese Patent Laid-Open No. 15625/1996 has described an optical system including: a light source for emitting a plurality of beams; a main scanning unit for collimating the plurality of light beams emitted from the light source, deflecting the light beams and mainly scanning the light beams on a photosensitive drum; and first and second lens systems each having lens power only in a subsidiary scanning direction and disposed between the light source and the main scanning unit; wherein the first and second lens systems are adjusted to move in a direction of an optical axis so that an image is formed from the plurality of incident beams by the main scanning unit while the interval between adjacent ones of the light beams on the photosensitive drum and the size of each of the light beams are adjusted to obtain a target light beam size.
Japanese Patent Laid-Open No. 109458/1997 has disclosed an optical recording apparatus including: a multibeam generating unit generating beams capable of being modulated in intensity independently; a rotary polygon mirror for collectively deflecting and scanning the plurality of light beams emitted from the multibeam generating unit, and scanning lenses for converging each of the light beams into a predetermined spot size on a scanning surface, wherein when resolution of a recording image is changed, a lens unit is newly provided in the optical system of the optical recording apparatus to adjust both the size of each of the image-forming beam spots and the scanning line interval in accordance with the resolution.
In the configuration described in each of Japanese Patent Laid-Open No. 101112/1992 and Japanese Patent Laid-Open No. 15625/1996, lenses disposed in the optical system are displaced to change the focal lengths of the lenses to thereby adjust the scanning line interval finely. Hence, there is a disadvantage in that the configuration cannot be adapted to the case where the scanning line interval varies largely as represented by conversion of resolution.
In the configuration described in Japanese Patent Laid-Open No. 109458/1997, the total magnification of the optical system in the main scanning direction and the total magnification of the optical system in the subsidiary scanning direction are changed equally by the newly provided lens unit. Hence, the chief rays of the beams are spread in the main scanning direction in the incident surface of the rotary polygon mirror, so that the beams are partially shaded by the edge of the reflection surface of the rotary polygon mirror. Hence, there is a problem that a wide scanning region cannot be retained. Moreover, when arrangement error, especially flap angle error occurs in the newly provided lens unit, there is another problem that imbalance easily occurs between the scanning line interval at the start of scanning and the scanning line interval at the end of scanning. In addition, there is a further problem that the scanning line interval cannot be adjusted independently in accordance with the resolution of the recording image.
The invention is attained in consideration of the problems in the related art and an object of the invention is to provide an optical recording apparatus for performing optical recording by forming an image from a plurality of beams on a photosensitive drum and scanning the plurality of beams, in which images different in resolution can be recorded without spoiling an effective scanning width and which has an adjusting unit for adjusting beam spots formed as an image on the photosensitive drum and the scanning line interval by an easy and tolerant adjusting mechanism, the adjustment being performed independently in accordance with resolution of a recording image.
To achieve the foregoing object, according to the invention, there is provided an optical recording apparatus in which a resolution converting lens system constituted by a spherical lens and first and second cylindrical lenses having lens power only in a subsidiary scanning direction is controlled to be detachably attached into an optical system newly so as to make it possible to attain a recording image different in resolution. On this occasion, the spherical lens is disposed in a position a satisfying the expressions (A) and (B)
a2+(HHS3+xcex4)xc2x7axe2x88x92(HHS3+xcex4)xc2x7fS3=0xe2x80x83xe2x80x83(A)
xcex4=(t1+t2)xe2x88x92(n1xc2x7t2+n2xc2x7t1)/(n1xc2x7n2)xe2x80x83xe2x80x83(B)
in which a is a distance from a position of a front focal point of rear one of lenses constituting abeam expander in the optical system to the position of the input side principal surface of the spherical lens, fS3 is the focal length of the spherical lens, HHS3 is the distance between the input side principal surface and the output side principal surface of the spherical lens, n1 is the refractive index of the first cylindrical lens, n2 is the refractive index of the second cylindrical lens, t1 is the thickness of the center of the first cylindrical lens, and t2 is the thickness of the center of the second cylindrical lens.
Further, in the optical recording apparatus, when resolution of the recording image is changed from xcex1 (dpi) to xcex2 (dpi), the resolution converting lens system detachably attached into the optical system converts the total magnification of the optical system to satisfy the condition represented by the expressions (C) and (D)
mmain less than mmainxe2x80x2xe2x89xa6(xcex1/xcex2)mmainxe2x80x83xe2x80x83(C)
msubxe2x80x2=(xcex1/xcex2)msubxe2x80x83xe2x80x83(D)
in which mmain is the total magnification of the optical system in a main scanning direction at resolution of xcex1 (dpi), msub is the total magnification of the optical system in a subsidiary scanning direction at resolution of xcex1 (dpi), mmainxe2x80x2 is the total magnification of the optical system in the main scanning direction at resolution of xcex2 (dpi), and msubxe2x80x2 is the total magnification of the optical system in the subsidiary scanning direction at resolution of xcex2 (dpi). In addition, in the optical recording apparatus, a rotating mechanism for adjusting the rotation of the resolution converting lens system around an optical axis is provided to make it possible to adjust a scanning line interval on the photosensitive drum, and the resolution converting lens system is disposed in a position where the rotation angle of the resolution converting lens system around the optical axis and the rate of change of the scanning line interval satisfy the expression (E)
0xe2x89xa6|xcex94Prate/xcex94xcex3| less than xc2xdxe2x80x83xe2x80x83(E)
in which xcex94 Prate is the rate of change of the scanning line interval in accordance with the rotation of the resolution converting lens system around the optical axis, and xcex94xcex3 is the rotation angle (deg) of the resolution converting lens system around the optical axis.
Incidentally, in the optical recording apparatus, when resolution of the recording image is changed from xcex1 (dpi) to xcex2 (dpi), the resolution of the recording image is converted by detachably attaching the resolution converting lens system into the optical system on the basis of an instruction given from a controller while changing both the rotational speed of a rotary polygon mirror and the modulating speed of the image-forming beam spots to (xcex2/xcex1) times, the rotary polygon mirror serving as one of constituent components of the optical system, the image-forming beam spots being scanned on the photosensitive drum.
Alternatively, when resolution of a recording image is changed from xcex1 (dpi) to xcex2 (dpi), a resolution converting lens system constituted by two cylindrical lenses having lens power only in a subsidiary scanning direction is controlled to be newly detachably attached to a light output side of rear one of lenses constituting the beam expander provided in the optical system. In this case, to convert the magnification of the optical system in the subsidiary scanning direction to thereby convert resolution of the recording image in the optical recording apparatus, the resolution converting lens system satisfies the expressions (F) and (G)
b=fS1+fS2xe2x80x83xe2x80x83(F)
|fS2/fS1|=xcex2/xcex1xe2x80x83xe2x80x83(G)
in which b is the distance between principal surfaces of the cylindrical lenses constituting the resolution converting lens system, fS1 is the focal length of front one of the cylindrical lenses constituting the resolution converting lens system, and fS2 is the focal length of rear one of the cylindrical lenses constituting the resolution converting lens system.
Incidentally, the resolution converting lens system detachably attached into the optical system is provided with a rotating mechanism for adjusting the rotation of the rotation converting lens system around the optical axis to thereby make it possible to adjust a scanning line interval on the photosensitive drum, and the resolution converting lens system is disposed in a position where the rotation angle of the resolution converting lens system around the optical axis and the rate of change of the scanning line interval satisfy the expression (H)
0xe2x89xa6|xcex94Prate/xcex94xcex3| less than xc2xdxe2x80x83xe2x80x83(H)
in which xcex94 Prate is the rate of change of the scanning line interval in accordance with the rotation of the resolution converting lens system around the optical axis, and xcex94xcex3 is the rotation angle (deg) of the resolution converting lens system around the optical axis.